Stream Live Prices

What this stream gives you. Open one HTTP connection and the server keeps it open, pushing each contract’s latest price, every new bid, and every trade as they happen. It uses server-sent events: a browser reads it with EventSource, and any HTTP client can read it line by line.

Request

GET /v1/predictions/stream?symbols=HORC_1126_Democratic,HORC_1126_Republican
GET /v1/predictions/stream?events=HORC_1126

Parameter	Description
`symbols`	Contract symbols, comma-separated or repeated (`symbols=A&symbols=B`).
`events`	Event IDs, comma-separated or repeated. Streams every `ACTIVE` contract under each event.

Send symbols, events, or both; duplicates are removed. A connection can carry up to 200 contracts, counted after events are expanded. Event IDs come from Search with type=events, or from a contract’s eventId. Get Event Contracts lists the contracts an event expands to. An event is expanded once, when you connect: a contract listed under it later is not added until you reconnect.

Events

ForecastEx takes buy orders only. Every order is a bid, for YES or for NO, so there is no ask.

Event	When	Payload
`subscribed`	First, once	Every contract on the stream, and each requested event’s contracts
`snapshot`	Once per contract, before anything else for it	Each outcome’s last trade, and the contract’s statistics
`bid`	Each new YES or NO bid	The order as the venue received it
`trade`	Each trade	One leg of a match
`stats`	Whenever the contract’s statistics change	The `contract` block alone
`gap`	When you fell too far behind	The contract whose bids or trades you missed. A fresh `snapshot` follows.

A : ping comment is sent every 15 seconds to keep idle connections open.

subscribed

{
  "symbols": ["HORC_1126_Democratic", "HORC_1126_Republican"],
  "events": { "HORC_1126": ["HORC_1126_Democratic", "HORC_1126_Republican"] }
}

events is present only when you asked for events.

snapshot

{
  "symbol": "HORC_1126_Democratic",
  "yes": { "last": "0.50", "lastSize": "1" },
  "no":  { "last": "0.50", "lastSize": "1" },
  "contract": {
    "high": "0.63",
    "low": "0.47",
    "settlement": "0.37",
    "volume": "123",
    "notional": "69.69",
    "openInterest": "54886",
    "tradingSession": "OPEN"
  },
  "receivedAtUnixMs": 1791499255775
}

Field	Notes
`yes`, `no`	Each outcome’s last trade price and size. `null` for an outcome that has not traded.
`contract.volume`	Contracts matched in the session. One match is one contract, even though it prints as two trades.
`contract.notional`	Total value traded in the session.
`contract.openInterest`	Open contracts.
`contract.high`, `low`, `settlement`	Session high, low, and settlement price.
`contract.tradingSession`	The venue’s state for the contract, such as `OPEN`, `HALTED`, or `CLOSED`.

The venue reports a contract’s last trade without saying which outcome it was for. The snapshot reads it as the YES price and sets NO to 1.00 minus it. After the first live trade, yes and no come from tagged trades.

bid

{
  "symbol": "HORC_1126_Democratic",
  "outcome": "NO",
  "price": "0.62",
  "size": "1",
  "orderId": "CZ1P1MS4A1C1",
  "placedAtUnixNs": 1791499265208000000,
  "receivedAtUnixMs": 1791499265211
}

A bid is sent when it is placed. Its later fills and cancellation are not sent, so a bid event does not mean the order is still resting.

trade

{
  "symbol": "HORC_1126_Democratic",
  "outcome": "NO",
  "price": "0.53",
  "size": "1",
  "tradeId": "CZKNTJJXE1DD",
  "aggressorSide": "Buy",
  "tradeTimeUnixNs": 1791499265208000000,
  "receivedAtUnixMs": 1791499265211
}

A match is a YES buyer and a NO buyer, so every match arrives as two trade events with the same tradeId: one for each outcome, at prices that add up to 1.00. Count matches by tradeId, not by event.

stats

{
  "symbol": "HORC_1126_Democratic",
  "contract": {
    "high": "0.63",
    "low": "0.47",
    "settlement": "0.37",
    "volume": "124",
    "notional": "70.16",
    "openInterest": "54887",
    "tradingSession": "OPEN"
  }
}

Each stats replaces the last. A match usually sends two: volume and notional change with the trade, and open interest follows. A new last price on its own sends no stats, because the trade events already carry it.

Contracts from events

For a contract you subscribed to through events, every snapshot, bid, trade, stats, and gap also carries eventId:

{ "symbol": "HORC_1126_Democratic", "eventId": "HORC_1126", "outcome": "NO", "price": "0.62", "size": "1" }

Ordering

For each contract, snapshot comes first, and nothing else is sent for that contract before it.
After that, bid, trade, and stats arrive in the order the venue sent them, and a match’s two trades come before the stats they moved.
Events for different contracts interleave.

If the contract is already being streamed, its snapshot is sent at once. Otherwise it arrives once the venue answers, usually within about a second.

Reconnecting

Events have no IDs and are not replayed. When you reconnect, as EventSource does on its own, you receive a new subscribed and a fresh snapshot for every contract. Bids and trades sent while you were disconnected are lost, but the snapshot’s prices and statistics show where the market is now.

Examples

cURL

curl -N "https://api.aries.com/v1/predictions/stream?events=HORC_1126"

Browser

const stream = new EventSource(
  "https://api.aries.com/v1/predictions/stream?symbols=HORC_1126_Democratic"
);

stream.addEventListener("snapshot", (e) => {
  const { symbol, yes, no, contract } = JSON.parse(e.data);
  console.log(symbol, "YES", yes.last, "NO", no.last, "volume", contract.volume);
});

stream.addEventListener("trade", (e) => {
  const { symbol, outcome, price, size } = JSON.parse(e.data);
  console.log(symbol, outcome, "traded", size, "@", price);
});

stream.addEventListener("bid", (e) => {
  const { symbol, outcome, price, size } = JSON.parse(e.data);
  console.log(symbol, "new", outcome, "bid", size, "@", price);
});

Working with the stream

Prices are decimal strings. "0.53", not 0.53. Parse them with a decimal type.
A missing value is null, not zero. A null last price means the outcome has not traded.
Do not put the stream behind a buffering proxy. Events must reach you as they are sent.

Errors

Errors are returned before the stream opens, in the usual error envelope:

Status	`code`	When
`400`	`MISSING_SYMBOLS`	Neither `symbols` nor `events` names anything
`400`	`TOO_MANY_SYMBOLS`	More than 200 contracts after events are expanded
`400`	`INVALID_SYMBOL`	A symbol containing whitespace, `*`, or `>`, or with a leading or trailing dot
`400`	`UNKNOWN_EVENT`	An event ID that does not exist
`400`	`NO_LIVE_CONTRACTS`	An event with no `ACTIVE` contract
`503`	`SHUTTING_DOWN`	The server is restarting; reconnect

An unknown symbol is not an error. It never receives a snapshot.

Historical Data

Reference Data

Live Data

Orders

Stream Live Prices

Request

Events

subscribed

snapshot

bid

trade

stats

Contracts from events

Ordering

Reconnecting

Examples

Working with the stream

Errors

​Request

​Events

​subscribed

​snapshot

​bid

​trade

​stats

​Contracts from events

​Ordering

​Reconnecting

​Examples

​Working with the stream

​Errors

Request

Events

subscribed

snapshot

bid

trade

stats

Contracts from events

Ordering

Reconnecting

Examples

Working with the stream

Errors